I am so sorry about her I hope it is not a new disease and whatever it is is treatable. I do not know much about diseases in fish but something that I can think of is how long have you been using the lake water? If you have used it her whole life there may be something new in the water that is eating her fins and scales. Try to see if some other type of water, or combination of, does not do that. If you have changed her type of water I personally do not know what to do other than see if there is somewhere that you can take her and see if they know what it is or can test it if she is almost gone (I hope she is not!). Hopefully she will get better soon.